1. Field of the Invention
This invention relates to objective lens systems having a soft focus function suited to photographic cameras or video cameras, and, more particularly, respectively to the introduction of spherical aberration into the objective lens system by axially moving a lens component constituting part of the system.
2. Description of the Related Art
Many photographic objectives that are made capable of occasionally producing soft focus effects so that pictures of soft feeling will be obtained, have been proposed. The production of the soft focus effect is attained by introducing into the optics controlled aberrations, mainly a certain type of spherical aberration. For example U.S. Pat. Nos. 4,013,347, 4,124,276 and 4,310,221 disclose photographic lens systems in which the amount of spherical aberration is varied to a large degree so that desired soft focus effects are obtained.
Of these, U.S. Pat. No. 4,124,276 creates an air lens between two lens surfaces which turn their concavities toward the point at which the principal ray at the full open aperture intersects the optical axis. To vary the soft focus effect, the two lens components, between which the air lens intervenes, are made to axially move at different speeds from each other. To effect focusing down to shorter object distances, the lens system as a whole is moved axially forward.
Because this method necessitates not only the provision of the air lens, which is not required for preserving the image quality in normal photography, but also the lens components which control the soft focus effect, and further because these lens components have to move not only when focusing is performed, but also at different speeds from each other when the soft focus effect is produced, the lens system and its operating mechanism tends to become very complicated in structure.
In U.S. Pat. No. 4,013,347, use is made of an optical member whose central portion is a parallel plate of no refractive power, and the marginal portion has increasing refractive powers as the radial distance from the optical axis increases. To obtain a desired soft focus effect, the optical member is axially moved. This method has a disadvantage that in order to obtain the soft focus effect, the optical member, which is unnecessary when in normal photography, must be newly provided.
In U.S. Pat. No. 4,310,221, by varying the air separation between the front component of positive power and the rear component of negative power, the soft focus effect is obtained, while focusing is performed by moving the lens system as a whole. This method had a disadvantage that the complexity of the structure o the operating mechanism for moving the components tends to increase.
Also, concerning the refocusing technique, U.S. Pat. No. 3,972,056 discloses the utilization of the output of the focus detector, but it has no description about the soft focus effect. Moreover, any concrete structure of the focus detector is not illustrated at all.
For uniformity of illumination with softening over the entire area of the image format, the marginal illumination has generally to be very largely increased when soft focusing.
In normal photography, however, contradistinctively, the marginal illumination is not necessarily as high, and such an increase in the marginal illumination rather causes the image quality to lower as the added light beam becomes flare. For this reason, the addition of the soft focus function to the photographic objective makes it very difficult to preserve the high grade of imaging performance when in normal photography.
Further, in soft focus photography, the spherical aberration introduced into the image format at any rate is so large that it is impossible for the human eye to carry out an accurate focusing even with the help of an optical range finder such as a microprism or a split prism. So, prior to take a shot with the soft focus effect, the photographer had first to set the lens system in the normal position to do focusing, and then to move the soft-focusing lens component.
The change from the normal to the soft focus position causes the image plane of sharpest focus to shift by a distance depending on the amount of spherical aberration introducible into the image format. Therefore, it has also been necessary to perform focusing again after the soft-focusing lens component is set.
Hence, the prior art has required for the photographer to re-adjust the position of the focusing lens component frequently until a desired soft focus effect is reached, being very troublesome and time consuming.